1. Field of the Invention
The present invention relates to a method of assuring data transmission and, more particularly, to a method of assuring data transmission between control units of a motor vehicle.
2. Prior Art
The invention is based on a method for assuring data transmission between control units in a motor vehicle, which are connected with each other by a data bus, in which one of the control units transmits a random bit sequence on the data bus, which is stored in all the control units, a key code is generated in all control units with the random bit sequence and an internal key, a data statement encoded with the key code is produced and transmitted by one of the control units, which is read and tested with the key code in the received control unit for its corrected encoding with the aid of redundant information contained in the encoded data statement.
A use of data transmission in which a particularly secure transmission of data between control units is required is known from DE 44 42 103. Said instance concerns an antitheft device for motor vehicles in which an immobilizer influences the release of a motor control device. In order to release the motor control device, a code must be input into a code reception device and the locking control-device must thereby be triggered. The data communication between the control units travels via an on-board bus. In the known prior art, the immobilizer is in fact set into operation by way of a key code that is only accessible to the user; the communication between the immobilizer control unit and a motor control unit takes place in an uncoded manner via the serial data transmission line. Since a releasing signal or locking signal of this kind is a very simple signal, which in the extreme case is comprised of one bit, such a signal can be manipulated very easily. It would be possible in such a method to feed in a manipulated signal by detaching a connecting line. A signal thus manipulated could easily be carried out by replaying a signal previously received and recorded from this transmission line.
The method according to the invention, with the characterizing features of the main claim has the advantage over the prior art that the data transmission between the control units takes place in an encoded form and that such a data transmission is not easy to manipulate. Thus detaching She connecting line and eavesdropping on the encoded signal transmitted via the line cannot produce successful results. Also, a replacement e.g. of the control unit for the immobilizer, as well as the replacement of the motor control unit does not produce successful results because the keys present in the devices no longer coincide. Also, the detachment and reconnection of battery voltage cannot influence the data transmission. Reading the memory of the control units and manipulating internal keys disposed in them also does not lead to any normal communication between the control units. Even the replacement in pairs of a motor control unit and a device for the immobilizer does not produce successful results since the key known only to the user is still lacking.
Advantageous improvements and updates of the method disclosed in the main claim are possible by means of the measures taken in the dependent claims. It is particularly advantageous that the communication between the control units in the form according to the invention is to be used for the security relevant communication between the control unit of an immobilizer and the control unit of a motor control.
Advantageously, the motor control obtains the random bit sequence, which represents the basis for the encoding, from sensor data present in the vehicle, which can be accessed via the data stream on the bus.
For security reasons, in order to prevent the random bit sequence from being guessed by trial and error, the random bit sequence should have a length of four bytes. Even for the length of the internal key, which is used for encoding messages between the control units, a length of at least 4 bytes is advantageous.
In order to produce a cryptogram with the key obtained, a signal byte is added bitwise modulo 2 to a first byte of the key, the result is subjected to a permutation and its result is then subjected to further additions or multiplications with the other bytes of the key byte. The resulting signal byte is comprised of the individual results of the encoding steps. The encoding method according to the invention can function with various permutation tables. In particular, the permutation can also be realized through multiplication. A increase in the security of the encoding is achieved by encoding steps that operate in parallel with one another.